1. Field of the Invention
The present invention relates to an anti-theft device for use in a vehicle or the like. More specifically, the present invention relates to an improved anti-theft device for performing communication operations with a transponder mounted in a key or card and for permitting a starting operation of the vehicle or the like when it receives a valid response code from the transponder.
2. Description of Related Art
In coordination with a mechanical key device of a vehicle, one known conventional anti-theft device performs code-based communication with an electromagnetic coupling transponder mounted inside a key via amplitude modulation. This anti-theft device allows the starting of the vehicle when the transponder of the key sends back a valid identification code. FIG. 5 shows one example of such a conventional anti-theft device.
The anti-theft device illustrated in FIG. 5 includes the transponder 2 mounted in a head portion of a key 1, a transceiver 5 which performs code-based communication with the transponder 2 via an antenna coil 4 that is wound around a key cylinder 3 into which the key 1 is inserted and a controller 6 for controlling the operations of the transceiver 5.
The transponder 2 reads an identification code stored in its EEPROM and subsequently encrypts the same identification code before sending it to the transceiver 5. The transponder 2 then receives a transmission code from the transceiver 5 and decrypts the same code before overwriting the identification code stored inside its EEPROM with the decrypted code. The above-mentioned device employs a rolling code format in which the identification code is changed every time the key 1 is inserted into the key cylinder 3.
Meanwhile, after receiving the identification code from the transponder :2, the transceiver 5, which is disposed in the vehicle side and which performs code-based communication with the transponder 2, verifies the received identification code in reference to an identification code it possesses and issues a permission command to activate an engine control unit (ECU) 7 under at least the condition that both identification codes match. After the controller 6 issues such permission, the ECU 7 begins the overall operations of an ignition system (e.g., driving spark plugs), an injection system (e.g., driving injectors) and a fuel supply system (e.g., driving a fuel pump).
The anti-theft device illustrated in FIG. 5 is provided with a key interlock solenoid 33 which mechanically inhibits a key switch (i.e., ignition switch) 32 from being switched from an "IGNITION ON" position to an "OFF" position to prevent the key 1 from being pulled out of the key cylinder 3 when a shift lever (not shown) is not at a "PARKING" position. A driver 8 controls the operation of the key interlock solenoid 33.
Meanwhile, regarding the overwriting of the identification code within the EEPROM of the controller 6 with the identification code from the transceiver 5, the overwriting of the identification code of the EEPROM of the controller 6 is not performed when the changed identification codes do not coincide due to factors such as incomplete overwriting of the identification code because the key 1 is pulled out, for example, while in the middle of overwriting the identification code of the transponder 2.
Thus, even with the adoption of the rolling code format for changing the identification codes of the transponder 2 and the controller 6, the overall level of safety and reliability of the anti-theft device itself deteriorates when the overwriting of the identification code of the controller 6 is not carried out.